The Astronomy Book

114

ENORMOUS

MASSES OF

LUMINOUS GAS

PROPERTIES OF NEBULAE

IN CONTEXT

KEY ASTRONOMER

William Huggins (1824 –1910)

BEFORE

1786 William Herschel

publishes a list of nebulae.

1850s Gustav Kirchhoff and

Robert Bunsen realize that hot

gases produce bright emission

lines in their spectra of light,

whereas cool gases absorb the

same wavelength, producing

dark lines in spectra.

AFTER

1892 Margaret Huggins is

made an honorary Fellow of the

Royal Astronomical Society.

1913 Dane Niels Bohr depicts

atoms as containing a central

nucleus surrounded by

electrons. Spectral lines are

produced when the electron

moves between energy levels.

1927 American Ira Bowen

realizes that the two green

lines caused by “nebulium”

are produced by oxygen atoms

that have lost two electrons.

I

n the 1860s, a pioneering

British astronomer named

William Huggins made

key discoveries by studying the

composition of stars and nebulae

using a spectroscope. This

instrument, a glass prism attached

to a telescope, splits white light into

its constituent light wavelengths,

producing a spectrum of color.

Gustav Kirchhoff and Robert Bunsen

had already noted the chemical

composition of the sun by studying

the dark absorption lines that occur

in its spectrum. These lines are

caused by the atoms of different

chemical elements absorbing

radiation at certain precise

wavelengths. Huggins, encouraged

by his astronomer wife, Margaret,

turned his attention deeper into

space, toward nebulae, the fuzzy

patches of light that had long

mystified astronomers. He used

spectroscopy to divide these

patches into two distinct types.

The spectra of nebulae

Huggins observed that nebulae,

such as the Andromeda nebula, had

a spectrum of light similar to that

of the sun and other stars—a broad

band of color with dark absorption

lines. The reason for this (not

discovered until the 1920s, after

Huggins’s death) was that such

Spectroscopes allow

astronomers to measure

a nebula’s spectrum

of light.

Some nebulae are found

to have spectra similar

to those of stars.

These nebulae are

enormous masses

of luminous gas.

Others have spectra that

emit energy at a single

wavelength.